And for every day of most years in Maine, the heat pump alone "just works". Cold this cold is very rare. Usually, the heat pump only goes out when the power goes out, in which case oil and gas heat goes out, too -- no power means no blower and circulator motors. Woodstoves have always been popular (though by no means ubiquitous) up here because the power does go out, because they're cheaper than fossil fuels, and because they're cozy. Lots of people have them even with oil and gas heat. Growing up, we used wood day-to-day and oil as a backup; many more people use the easier heat day-to-day and light the fire on occasions when they want to gather 'round it.